In the manufacture of steel, furnaces are often lined with refractory material. This refractory material wears as a result of the process. After the refractory material has worn down to a predetermined level, it must be replaced, otherwise a dangerous situation may arise wherein the molten steel will leak from the furnace. Steel making equipment is quite expensive and the economic efficiency of such equipment is dependent upon the length of time a furnace can be employed without shutting down for providing a new refractory lining. Therefore, it is of substantial economic significance to employ a refractory lining as long as possible, yet it is also important not to allow the lining to be used when it has worn below a safe minimum thickness.
Presently, several methods are employed for monitoring the thickness of refractory linings in steel making equipment.
One of the methods that has been employed is stereophotography which utilizes changes in parallax between a series of photographs to determine the dimension and changes of dimension in a furnace, mine or other object. A careful and time consuming comparison of photographic images makes the stereo method a cumbersome and expensive process control. In addition, an accuracy of only between 30 to 40 millimeters is achieved.
Another prior art method comprises the embedding into the furnace lining precise deposits of radioisotopes which have predetermined concentration and composition characteristics. The radioisotopes characteristics vary as a function of the distance from an original surface, which during erosion recedes releasing a predetermined concentration of radioisotopes. Appropriate monitoring equipment detect the erosion of the surface. The disadvantages of the isotope method include limited applicability to a specific area or areas. Hopefully, these areas are representative of the erosion of the entire surface to be monitored. In order to alleviate the abovedescribed disadvantages, a great number of unique isotope characteristics must be employed on a great number of different portions of a surface which may provide a variety of indications which due to their complexity may even have a tendency to mask the true erosion of the entire surface. However, if the area is small only a single or limited measurement sample is obtained which usually is not representative of the entire surface. In addition, the measuring time is prohibitively long and expensive since elaborate isotope measuring and evaluation techniques are required. Therefore, the isotope method would only be useful as an aid in checking and calibrating other methods. Sonic methods usually are precluded because of thermal gradients and high refractions in the atmosphere.
Distance measuring equipment such as electro-optical type would be ideally suited for measuring thickness in a furnace since one can do that from a distance. However, furnaces are moved during their usage and it would be impossible to maintain a constant relationship between the electro-optical distance measuring equipment and the furnace and, further, since the interior of the furnace is at an extremely high temperature level, it would be difficult, if not impossible, to site the electro-optical distance measuring equipment on a particular point for separate readings.